Implementing Atomicity for 3 separate exercise API calls involving different parties

Hi, I had a question regarding implementing atomicity for the below flow.

We have two contracts: A and B.
Contract A’s signatory is: Operator
Contract B’s signatory is: User

Through API calls from a Node.js backend, we want to achieve the following flow:

1. Operator exercises choice on Contract A.
2. User exercises choice on contract B.
3. Operator exercises choice on Contract A.

Can we make these 3 steps atomic somehow?

Thanks!

To get a single Daml transaction and therefore atomicity, you also have to make a single command submission. Multi-command submissions don’t really help here to do the different exercises since they have different controllers and you almost certainly cannot expect to have actAs = [user, operator].

So your command submission has to be a single exercise. Given that the operator seems to initiate this process they should be the controller. In the body of that choice you can then do the 3 operations you suggested. To make authorization work out the user needs to delegate their authority to the operator.

So in the end, you end up with a template like this:

template Helper
  with
    user : Party
    operator : Party
    contractA : ContractId A
    contractB : ContractId B
  where
    signatory user
    observer operator
    nonconsuming choice DoStuff : ()
      controller operator
      do exercise contractA FirstChoice -- operator is controller
         exercise contractB SecondChoice -- user is controller
         exercise contractA ThirdChoice -- operator is controller

If having the user delegate their authorization by creating such a contract is not an option, I don’t think you can make this atomic.

assuming operator is not a stakeholder of B, for the above to work one needs to be able to at least readAs user to fetch contractB, i.e. something like

submitMulti [operator] [user] do exerciseCmd helper DoStuff

would work. However if operator is set as an observer of B DoStuff would work with just

submit operator do exerciseCmd helper DoStuff

correct?

Good point, yes you need to make the contract visibile via observers or readAs.

Thanks @cocreature and @davide_ooz for the input! Will give it another go